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3.5.1 • Public • Published


This is a fork of https://www.npmjs.com/package/memfs, since there's a bunch of critical bugs and missing maintenance there, which might not be a priority for that project, but are definitely a priority for me (though I'm sending PR's). Upstream memfs has nearly 12 million downloads a week, so I can see why doing these changes would be really scary there!

Things I've done here include:

  • update to newest jest, and increase maxWorkers for testing; fix an issue with one test
  • update to newest typescript and node typings
  • update target to es2020 from es5; this does make code run more efficiently and is much easier to debug if you want to directly edit node_modules
  • fix the 13 security vulnerabilities revealed by npm audit
  • upgrade prettier and switch to using the defaults
  • switch to package-lock.json instead of yarn's lock
  • fix chmodSync bug
  • fix utimesSync bug
  • fix realpathSync bug

See https://github.com/sagemathinc/memfs-js

Upstream description

In-memory file-system with Node's fs API.

  • Node's fs API implemented, see old API Status, missing list, missing opendir
  • Stores files in memory, in Buffers
  • Throws sameish* errors as Node.js
  • Has concept of i-nodes
  • Implements hard links
  • Implements soft links (aka symlinks, symbolic links)
  • Permissions may* be implemented in the future
  • Can be used in browser, see memfs-webpack


npm install --save memfs


import { fs } from "memfs";

fs.writeFileSync("/hello.txt", "World!");
fs.readFileSync("/hello.txt", "utf8"); // World!

Create a file system from a plain JSON:

import { fs, vol } from "memfs";

const json = {
  "./README.md": "1",
  "./src/index.js": "2",
  "./node_modules/debug/index.js": "3",
vol.fromJSON(json, "/app");

fs.readFileSync("/app/README.md", "utf8"); // 1
vol.readFileSync("/app/src/index.js", "utf8"); // 2

Export to JSON:

vol.writeFileSync("/script.sh", "sudo rm -rf *");
vol.toJSON(); // {"/script.sh": "sudo rm -rf *"}

Use it for testing:

vol.writeFileSync("/foo", "bar");
expect(vol.toJSON()).toEqual({ "/foo": "bar" });

Create as many filesystem volumes as you need:

import { Volume } from "memfs";

const vol = Volume.fromJSON({ "/foo": "bar" });
vol.readFileSync("/foo"); // bar

const vol2 = Volume.fromJSON({ "/foo": "bar 2" });
vol2.readFileSync("/foo"); // bar 2

Use memfs together with unionfs to create one filesystem from your in-memory volumes and the real disk filesystem:

import * as fs from "fs";
import { ufs } from "unionfs";


ufs.readFileSync("/foo"); // bar

Use fs-monkey to monkey-patch Node's require function:

import { patchRequire } from "fs-monkey";

vol.writeFileSync("/index.js", 'console.log("hi world")');
require("/index"); // hi world


See also

  • spyfs - spies on filesystem actions
  • unionfs - creates a union of multiple filesystem volumes
  • linkfs - redirects filesystem paths
  • fs-monkey - monkey-patches Node's fs module and require function
  • libfs - real filesystem (that executes UNIX system calls) implemented in JavaScript


Unlicense - public domain.


npm i @cowasm/memfs

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  • wstein